Non-impact recording methods are becoming popular for making a hard copy image of electronic information due to the fact that less noise is produced in recording compared to impact recording. Also, ordinary paper can be used for recording without the need for any special treatment, such as photographic fixing.
In one ink-jet method which has been put into commercial use, a pressure pulse is applied to the ink during recording to jet the ink from an orifice in the recording head. However, a small-sized ink jet recording device cannot be used for such method. Further, in order to perform printing with the necessary ink density, mechanical scanning has been required for the ink-jet device. As a result, in such conventional ink-jet method, high-speed ink-jetting has not been attainable.
Recently, several techniques have been proposed to eliminate the aforementioned defects and make high-speed ink-jetting possible. In one proposed technique, a magnetic field is applied to magnetic ink positioned in the vicinity of a magnetic electrode array to produce a meniscus on the surface of the maqnetic ink. There is produced an ink jetting condition corresponding to a desired ink density, and an electrostatic field is applied to the magnetic ink to cause the magnetic ink to jet from the recording head. Although the magnetic ink-jet method has an advantage in that higher-speed recording can be performed using electronic scanning, the method has a disadvantage in that color imaging becomes difficult because of the effect of the color of the magnetic material in the ink.
In another proposed technique, the plane ink-jet method, ink is disposed in a slit-like ink reservoir parallel to an electrode array and is caused to jet out in accordance with an electric field pattern formed between an electrode array and an electrode opposite to the electrode array, with recording paper interposed therebetween. Although the plane ink-jet method has an advantage in that a small orifice is not required and, therefore, the problem of ink clogging of the orifice is avoided, the method has a disadvantage in that a high voltage is required for making the ink jet. In the method, it is necessary to perform time-division driving of the electrode array in order to prevent voltage leakage between adjacent electrodes. As a result, in the plane ink-jet method, high-speed ink jetting cannot be carried out satisfactorily.
Further, a so-called thermal bubble jet method has been proposed. Thermal energy is used to jet ink from an orifice. In the thermal bubble jet method ink is rapidly heated to produce surface boiling in the ink so as to rapidly form bubbles within an orifice and the ink is jetted out due to the increase of pressure within the orifice. In this method, it is required to rapidly raise the temperature of a heating element to produce surface boiling. Accordingly, the method has a practical disadvantage in that thermal transmutation of ink occurs and thermal degradation of the protective layer on the heating elements often occurs.
Prior to the present invention, the present inventor has proposed a novel high-speed, ink-jet method in which the most important defect in the conventional ink-jet method, that is, the low speed, is improved and in which the defects in the high-speed ink jet methods described above are avoided. This novel high-speed ink-jet method is called the thermal electrostatic ink jet method, in which thermal energy is applied to ink while simultaneously or successively applying an electrostatic field to the ink to cause the ink to be jetted.
The thermal electrostatic ink-jet recording head used in such a thermal electrostatic ink-jet method comprises heating elements for applying thermal energy to the ink, an electrostatic induction electrode applying an electrostatic field to the ink, and means for feeding to and holding the ink in an ink orifice to facilitate jetting of the ink.
More particularly, the proposed recording head comprises a first plate member formed of an insulating substrate having an array of heating resistors formed thereon and composed of a plurality of heating resistors disposed at predetermined intervals, a second plate member formed of an insulating substrate and disposed opposite to the first plate member at a predetermined distance apart, a slit-like opening formed betweeen the first and second plate members, a means, including a pump or the like, for feeding ink to and holding ink in the slit-like space, and an electrostatic induction electrode disposed on one of the plate members to apply an electrostatic field to the ink.
As the result of a further study of the aforementioned recording head proposed by the inventor, the inventor found that the wetability, by the ink at the ink orifice, of the surfaces of the first and second plate members adjacent the slit greatly controls the form, the maintenance, and the stability of the ink meniscus at the ink orifice and exerts a great influence on the ability to provide a stable, uniform recording operation.